Presently, instruments for along-the-cable underwater profile measurement are basically categorized into two types:
The first type is an instrument that adopts an oil sac for buoyancy drive control and enables the entire moving platform to produce buoyancy or gravity through adding pressure to and relieving pressure from the oil sac. Specifically, the mobile sealed compartment is connected with an outer oil sac which is connected with an inner oil sac via a solenoid valve and an oil pump is pressurized by an electronic circuit controlled air pump in the compartment to discharge the oil from the inner oil sac to the outer oil sac so as to increase the volume of the drain water from the platform, thus increasing the buoyancy of the platform to make the platform ascend; on the contrary, when the oil returns from the outer oil sac to the inner oil sac owing to seawater pressure, the platform will descend owing to the decrease of the buoyancy of the entire platform. Although this type of instrument can easily control, through its circuit, the slight buoyancy of the platform and change the buoyancy without additional electric power consumption during platform motion, it has the following challenges:                1. The mechanical control part in the compartment is complex and requires a precision air pump, oil pump, solenoid valve and accurate measurements with related sensors; moreover, this type of instrument requires many batteries and the system is complex and bulky.        2. All power is sourced from batteries which have a huge influence on the gravity of the sealed compartment, the design of compartment body is closely related to the quantity of batteries, their service life directly determines the overall design, that is, the battery may affect the whole instrument;        3. This type of instrument, with limited working water depth, is mostly applied to the measurements for the water depth less than or within 2000 m at present, and for the water depth beneath 2000 m, the power consumption of the batteries required for pumping the oil to the outer oil sac owing to the increase of water depth cannot keep balance with the battery efficiency so that the overall design of the compartment cannot satisfy further power consumption, thus resulting in failure to complete further measurements.        
The second type is an instrument that adopts a motor to drive a gear to climb along a cable. The instrument works as follows: A moving platform is seized onto the underwater cable via a roller, the platform buoyancy is set to zero, the motion of the platform along the cable is driven by the motor in the electronic compartment, the motor drives the gear to rotate, thus driving the roller outside the compartment to move up and down along the cable and the ascending and descending of the platform are controlled through controlling the rotation direction of the motor. The instrument has the advantages including simple mechanical design part, minor influence of the water depth and simplicity in electronics control but has several difficulties:                1. Every step of platform motion will consume electricity, which constitutes a huge challenge to the batteries; like the first type of instrument, there are corresponding requirements on the design of the electronics compartment, the times of the working profiling determines the quantity of the batteries, thus determining the design size of the electronics compartment.        2. The platform, with limited motion speed, requires the motor for every meter of motion in the process of ascending and descending, and huge electricity consumption is required with the increase of the platform motion speed, which also constitutes a huge challenge to the reliability of the inner motor and outer roller.        
It can be seen that the power energy storage mechanism (battery and motor) and measurement mechanism are set on one platform for the above-mentioned two measurement methods so that there is a problem that the energy for controlling the platform motion is restricted by the battery, thus exerting a decisive influence on the overall design.